Resonant cavities can be used as spectral filters of electromagnetic waves (e.g., radio frequency and microwave frequency signals). For example, different communication channels in a telecommunications system can have different channel frequencies, so that signals on the different channels do not interfere. Typically, each channel of a transmitter or a receiver in the telecommunications system includes a narrow bandpass filter to select the frequency of the signal to the channel frequency.
The bandpass filter can include a resonant cavity, the spectral response of which is determined by the dimensions and the electromagnetic properties of the cavity. The resonant cavity can include resonators having any shape. The positions, size, and shapes of resonators within a resonant cavity are selected to tune the spectral response of the cavity to a desired response. The accuracy to which the dimensions of the resonators are manufactured, the shape, the surface finish, surface conductivity and the accuracy with which the resonators are located in the cavity are important factors in determining the spectral response of the cavity. Often cylindrically-shaped resonators are used in a resonant cavity because a cylindrical shape is useful for handing high concentrations of electromagnetic power within the cavity without arching. Good electrical contact between a resonator and the walls of the cavity ensures that the cavity operates as designed.